The development of new drugs, formulations and other systems for administration of physiologically active peptides, proteins, organic drugs, other therapeutics and materials is driven by the need to achieve the desirable physiological effects. With respect to peptides and proteins, many of them have been observed to be unstable in the gastro-intestinal tract and therefore may need to be stabilized or protected or delivered via systemic circulation. In addition, peptides and proteins that have low molecular masses tend to have short biological half-lives due to their efficient removal from systemic circulation via kidneys. For example, a fraction of these peptides and proteins can also be removed via reticuloendothelial uptake due to recognition by monocyte/macrophages or as a result of opsonization by complement components. Many peptides and proteins can also lose their activity in vivo due to proteolysis (peptide bond cleavage).
In part to circumvent these undesirable effects, a drug delivery system may be used. There are several drug delivery strategies that can be useful for peptide and protein delivery in vivo. First, a continuous systemic infusion of drug via a pump can be employed. This strategy has proven efficient in clinical practice but may be impractical for outpatients requiring high levels of mobility. This strategy also has associated disadvantages including quality of life and potential intravenous (I.V.) line infections.
Second, peptides and proteins can be included in an implantable pump comprised of a capsule with a membrane allowing diffusion of the drug, for example, at a desirable release rate. Due to limited volume of these capsules, peptides and proteins are often used in a concentrated formulation which leads to a loss of solubility due to aggregation and potential loss of specific activity. In most cases, the drug is usually released into the extracellular space and distributed in lymphatics. Overall concentration of peptide or protein may be affected by local lymph node activity and the efficacy of lymph node drainage of the implantation site. There is also a potential of host reaction to capsule material but in general, this side effect is infrequent.
Third, the drug release system can be made biodegradable as a result of encapsulation or inclusion into degradable drug delivery vehicles or carriers, e.g. polymeric matrices, particles or membrane vesicles (liposomes). These delivery systems are usually either implantable or injectable. Implantable drug delivery systems are often placed under the epidermis where the components of the system are usually slowly degraded as a result of biological activity of surrounding cells (i.e. as a result of the release of enzymes degrading chemical bonds that hold these implants together).